Reference electrodes are electrochemical half cells providing a maximally constant reference voltage. The reference electrode may form an independent unit or may be part of a combination electrode, for instance of a single-rod measuring chain. Besides pH electrodes and ion-selective electrodes, measuring systems for coulometry and polarography are also suitable electrochemical measuring systems.
If the ion-conducting transition between the reference electrode and measured medium takes place through a porous diaphragm, the function of the reference electrode may be impaired by contamination of this diaphragm. The contamination may generate undesirable potentials on the diaphragm, may increase the resistance of the diaphragm, prolong the response time of the reference electrode or impart to it an undesirable selectivity.
Contaminations of the diaphragm may be caused by ingredients of the measured medium, but also by precipitations in which ions from the interior of the reference electrode participate. Silver chloride, relatively soluble at high KCl concentrations, may for instance precipitate on the diaphragm, because the KCl concentration is diluted there by measured solution diffusing into it. The function of the reference electrode may also be impaired by ion precipitations from the reference element with anions from the measured solution, for instance silver sulfide.
To assure a flawless function of the reference electrode and to avoid contaminations in which cations of the reference system, for instance silver ions from the interior of the reference electrode, participate, it is desirable to prevent the migration of such cations to the diaphragm. Toward this objective, various devices have already been developed, which, however, provided only an unsatisfactory solution to this problem. Examples are
a) devices reducing the ion transport by convection; PA1 b) devices reducing the diffusion by reversible binding or ion repellency; and PA1 c) devices binding the silver ions or silver halogenide complexes irreversibly.
The devices mentioned under (a) include those where the migration of cations to the diaphragm caused by convection, for instance of silver ions, is prevented by providing a diffusion gap between the discharge wire of the reference electrode and the diaphragm. A pertinent example is a single-rod measuring chain developed by the applicant, where, for this purpose, a cotton section is installed. Moreover, known from the German patent document 32 28 647 is the use of a gel- or polymer-fixed reference electrolyte, and from the British patent disclosure 20 93 194 the installation of a bridge electrolyte with a porous diaphragm as a transition to the reference electrolyte.
The effectiveness of the said devices is limited though, since silver ions can in time reach the diaphragm by diffusion.
The devices named under (b) include, for instance, the device described in German patent document 33 05 962, where the diffusion of silver ions to the diaphragm is retarded by installation of ion, cation or anion-cation exchangers to which silver halogenide complexes, for instance (AgCl.sub.4).sup.3-, are reversibly bound Moreover these devices include the one known from the British patent disclosure 20 93 193A, where the diffusion is reduced by installation of a microporous polymer to the porous walls of which sulfone groups are bound, since these sulfone groups repel the negatively charged (AgCl.sub.4).sup.3- complexes.
The effectiveness of these devices is limited because the diffusion of silver ions to the diaphragm is not prevented but only reduced. The effectiveness is poor particularly at high temperatures, because the diffusion velocity of the ions increases greatly with the temperature. Thus, it has been found by measurements that a device according to the British patent disclosure 20 93 193A allows, at 80.degree. C., the passage of 38 times as many silver ions as at 25.degree. C.
Contrarily, a complete prevention of the silver ion diffusion to the diaphragm was accomplished by means of the devices named under (c), by installation of ion exchangers to which the silver ions or silver halogenide complexes are irreversibly bound. Devices of that type have been described in German disclosures 34 15 089 and 39 34 302.
Since the effect of these devices is based on a surface effect, their capacity is limited. The disadvantage of these devices is thus to be seen in their limited service life, because--once all of the binding locations of the ion exchanger acting as ion barrier have been occupied--the following cations, for instance silver ions, can diffuse through this ion exchanger to the diaphragm.
The problem underlying the invention, therefore, is to provide a reference electrode where the migration of cations stemming from the interior of the reference electrode, for instance of silver ions, to the diaphragm and consequently its contamination is prevented.